This invention relates to high pressure gaseous discharge devices and in particular to improvements in the envelope structure and window seal for high pressure arc lamps.
In any arc lamp, various portions of the lamp structure will be at different potentials corresponding to one or another electrode. It is necessary, therefore, to provide insulation between these portions in order to maintain their mutual electrical isolation. Prior art high pressure arc lamps, for example, as described in U.S. Pat. Nos. 3,876,908; 3,852,629; and 3,808,496, employ ceramic cylinders separating the lamp envelope into anode and cathode portions with a circumferential surface of the insulator as part of the lamp envelope exterior. It will be apparent that as a consequence of this construction, the lamp design, operation and usage are subject to a number of critical specifications.
One such critical specification is related to the internal pressure and temperature to which the ceramic and the ceramic to metal bonds are subject. Ceramic cylinders or rings so employed are primarily under tensile stresses due to the high pressure gas filling of the lamp which tends to push apart the anode and cathode portions of the lamp envelope. The brazed metal to ceramic bonds of these prior art lamps are likewise under tensile or shear stress. Due to the properties of the brazing alloys and the metallic members, the stresses within the joint must be maintained within safe tolerances. These stresses are thermally dependent and thus place limits on the lamp operation and associated cooling system.
Another critical consideration is the ambient environment wherein the lamp will be operated. Moisture, dirt, grease, fingerprints and the like can seriously degrade the insulation characteristics of the ceramic, especially for high voltage starting conditions. Moreover, the exposed ceramic is subject to impact which can initiate a fracture thereof.
It will be clear also that failure of the ceramic or the metal-ceramic seals of a high pressure lamp can be hazardous to personnel or equipment nearby.
Another prior art arc lamp, described in U.S. Pat. No. 3,715,613, employed a ceramic cylinder with a recessed plane surface for accepting a metal disk which in turn supported the anode structure. The outside surface of the ceramic was brazed to the inner surface of the envelope neck. Thus, the metal ring exerted compressive and shear stresses at one end of the ceramic due to the internal gas pressure while a brazement of the outer circumferential wall of the ceramic to the inner cylindrical surface of the envelope neck resulted in shear stresses due to the internal gas pressure.
An arc lamp is also characterized by an optical window which must withstand a very great pressure differential between the interior and exterior of the lamp. Prior art window seals in such lamps have normally employed a window disc of sapphire, metallized around its circumference and then brazed to one leg of a U-shaped metal flange, which was subsequently brazed into the window-cathode portion assembly, as shown for example in U.S. Pat. No. 3,852,629. This window sealing technique resulted in an improved seal but one which is not as good as the seal to be hereinafter described with respect to this invention.